(Liquid + liquid) equilibrium (LLE) data for (water + propionic acid + diethyl phthalate) were measured under atmospheric pressure and at T = (298.2, 303.2, 308.2, and 313.2) K. Phase diagrams were obtained by determining solubility and tie-line data. The UNIFAC model was used to correlate the experimental tie-line data. The average root-mean-square deviation between the observed and calculated mass fraction was 0.03. Distribution coefficients and separation factors were evaluated over the immiscibility regions. (c) 2005 Elsevier Ltd. All rights reserved.